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Microfluidic Viscometer using ExiGo pumps

Viscosity of a liquid is an important parameter which helps manufacturers predict how a material will behave in the real world and this latest blog introduces a new method from Hintermüller et al. [1], on measuring viscosity using Cellix’s ExiGo syringe pumps.

Unfortunately, viscosity is not so easy to measure…but first, what is viscosity and why is it important?

What is Viscosity?

If you want to look at the real behaviour of fluids, then you need to understand viscosity. There are various definitions of viscosity including the internal friction of a liquid; a fluid’s ability to resist flow or resistance to deformation under shear stresses. As a formula, it is described as:

Viscosity = shear stress / shear rate

When describing viscosity in terms of a liquid, you may hear people describe a liquid as “thin” which is more water-like and these liquids typically have a low viscosity. You’ve probably heard the phrase “thinning out paint” and this is where paint is watered down. In contrast, “thick” liquids such as honey or syrup are those with high viscosity.

Viscosity measurements are used in many industries including:

  • Food – to maximise production efficiency and cost effectiveness; e.g. how toothpaste flows out of a tube or ketchup out of a jar.

  • Oil – to determine the effectiveness of lubricating oil or in the case of crude oil, viscosity determines our ability to pump it out of the ground.

  • Cosmetics – to determine the texture, feel and flow of cosmetic products.

  • Chemical and biological processes monitoring.

So, what’s the problem with measuring viscosity?

One of the first methods for measuring viscosity was introduced in the 1930s and this involved measuring the time it takes for a volume of fluid to flow under gravity through a calibrated glass capillary. Since then, other methods have been introduced including using rotating concentric cylinders or plate configurations. But one of the main challenges is achieving high accuracy and precision. Why is this important? Because even an error of just 1% can cause blend adjustments that easily result in increasing product cost by a penny per gallon. In economic terms, this can could cost a large lubricant manufacturer $1 million or more in lost revenue per year, [2]. Also, many existing commercial methods lack the possibility for in-line measurements and require large sample volumes; the latter rendering them difficult to use for biological samples where often only small sample volumes are available. As a result, in recent years, microfluidic solutions have been developed which include methods to detect the interface position in a microfluidic channel between co-flowing liquids (one sample liquid and one reference liquid). The problem with this is that optical methods are usually used to determine this interface position – this requires colouring the liquids or adding tracer particles to increase optical contrast.

New solution

A potential solution to these methods is presented by Hintermüller et al. which describes a microfluidic viscometer with an integrated capacitive sensor. The capacitive measurement is achieved by interdigitated electrodes which are screen- printed onto the bottom of the chip.

Advantages to this method include:

  • No addition of coloured dyes or tracer particles required

  • Both fluids may be conductive and/or insulating,